Signal selection by amplitude discrimination



Feb. 4, 11. P. M. HAFFCKE SIGNAL SELECTION BY AMPLITUDE DISCRIMINATION s Sheets-Sheet 1 2 Filed June 28, 1938 OUTPUT INPUT SIB.

v INVENTOR Philip M. 'Haffcke ATTORNEY Feb. 4, 1. P. M. HAFFCKE SIGNAL SELECTION BY AMPLITUDE DISCRIMINATION Filed June 2a, 1938 3 Sheets-Sheet 2 HHHHI 1 1 l I l l I ENVELOPE F 4; g I II F ENVELOPE H D 1 Z I I I 4 l \A I Illllaaf. 1 J 1 z I I I I INVENTOR Philip M. Haffcke ATTORNEY Feb.4,1941. RQHAFFCKE 2,230,243

SIGNAL SELECTION BY AMPLITUDE DISCRIMINATIOF Filed June 2a, 1938 s Sheets-Sheet 3 I I] @ZE] ATTORNEY.

Patented Feb. 4, i941 UNITED STATES PATENT OFFICE SIGNAL sanac'rrox nr AMPLITUDE mscnmmn'rron Philip m. mom, Washington, D. c.- Application June as. less. Serial No. ziemfs chum. (on. 250-!) (Granted under the act of March 3, Q'isas, as amended' April 30, 1928; 370 0. 6.157)

This invention relates broadly to radio communication wherein the desired signal is sep rated by amplitude discrimination from other energy received simultaneously therewith, such as low. level noise and high amplitude static disturbance, whether on the same frequency or anbut with a slope opposite the slope of the first mentioned limb.

Among the numerous objects of this invention are:

To separate a desired signal from accompanying non-signal energy at twice the fundamental signal frequency, by amplitude differentiation;

To cause those portions of waves below chosen amplitude limits to be ineffective to change the output of the vacuum tube and those of greater than a chosen upper amplitude limit to be ineffective by cut-off of plate current, while those portions of the last mentioned waves that lie within the said limits are thrown into higher harmonics;

To provide for secrecy by blanketing message signals, sent at a chosen amplitude, with signals at the same or other frequencies having different amplitudes and which cannot be separated from the message by an ordinary receiver;

To separate a message signal when sent on an envelope defined by breaking up an envelope of different amplitude into supersonic groups;

Toprovide for receiving a message intelliglbly when blanketed by interference of a different amplitude.

In the drawings:

Fi 1 shows schematically a vacuum tube network adapted to practice the present invention;

Fig. 2 depicts an E -I curve characteristic of the network of Fig. 1, and certain outputs derivable therefrom;

Fig. 3 is a curve showing the variation of double frequency output in relation to input.

Fig. 4 illustrates changes in the output when the fixed grid biasing potential is shifted to a position different from that in Fig. 2;

Fig. 5 shows two different signal envelopes on {the samecarrier, separable by the present invenon; A

Fig. 6 is generally similar to Fig. 5 but-here each of the two envelopes of equal amplitude is produced by, say,- a 1000 cycle tone, of equal volume with the minimum of one envelope substailntially coinciding with the maximum of the ot er;

Fig. 7 illustrates the productionoff an effect similar, for practical purposes, to that in Fig. 6, by breaking up one envelope into" supersonic In Fig. 8 the second envelope, which carries the message is blanketed by a first envelope of less power and a third envelope of much greater 15 power;

Fig.9 shows the effect of adjusting the circuit's constants to obtain a more sharply pointed Eg-Ip curve and operating withthe grid bias at one limit'of the saturation current portion of the20 curve;

Fig. 10 presents a group of EgIp curves resulting from altering the circuit values;

Fig. 11 shows graphically the response under the conditions of operation illustrated in Fig. 9; 25

Figs. 12 to 14 and 18 depict arrangements that efficiently reduce the effects of undesirable intratube coupling when operated in conformity with the present invention;

Fig. 15 shows means for reducing the average 30 power output by chopping a fundamental frequency wave with a harmonic thereof;

Fig. 16 illustrates the relation of the fundamental frequency and the harmonic by which the fundamental is "chopped by the system of 5 Fig. 15;

Fig. 17 discloses an Er-Ip characteristic curve and the relation of the input and output voltages about certain points thereon;

Figs. 19 to 21 show various forms of Er-Ip o curves upon which thermionic tubes may be operated for specific purposes.

. Fig. 22 shows means for reducing the average power output by chopping.

The present invention is based upon discoveries 45 made while studying the action of tubes and networks such as are disclosed in my Patent 2,106,172. It is shown in this patent that if one or more grids, preferably adjacent the anode of the tube, be given a positive bias and connected o through suitable resistances to the cathode they will intercept electrons from the electron stream andduring excessive flow of electrons resulting from heavy surges of received energy the flow of current through the above mentioned resistances will reduce the potential on the grids and cut down the stream of electrons to the anode. even to the point of causing cessation of anode current fiow when the circuit values are properly adiusted.

Fig, 1 shows schematically a vacuum tube net.- work adapted'to practice the present invention, wherein. the input transformer I2 is connected to grid-ls and cathode llofitube II. The interceptor. grids id and Hare-connected inseries with resistances wand l9respectiveiy and also to a source of positive-potentiah The output circuit between anode 20 and cathode l4 includes a tunedontputatransformer 21 to pass a chosen fundamental.frequem'zy;and-also a like transformerr22 tunedtoxpasszdouble the fundamental frequency. Screengrid; .23} is. through a filter choke -24 to a voltage divider 29 that also serves as the supply to anode 29.

Fig.2 presents an Er-1p curve 21 thatis characteristic of the tube II when operated with the proper circuit values, this curve consisting of a limb 28 rising from cut-oil to a straight fiat top portion 29 representing saturation current and an oppositely sloping limb 90 descending to cut-oil. If the voltage divider 29 be so adjusted as to impress upon signal grid I! a biasing potential represented 'by the line designated bias in Fig. 2, that is, having a value represented by substantially the midpoint of portion 29-0! curve 21, the results'next to be described will be attained.

An input voltage wave of amplitude not exneeding one-half the length of portion 29 is indicated at 3| and it will be noted that since it lies within the voltage limits corresponding to saturation current it will have no eflect on the output current as is seen by comparing the areas represented by i-2-9 in, the output, corresponding to the points l-2-9 on voltage wave 3!. The curve 32 represents an input voltage wave having an amplitude lying between the cut-oi! terminal of limbs 29 and 39 of curve 21. It will be noted that those portions of the voltage wave 92 lying within thespace designated A, that is, between the limits of the portion 29, have no effect upon the output but the portions oi the wave lying between the termini of limbs 29 and 30 do aifect the output, as indicated by-the spaces 4-5-8 in the output corresponding to the points 4-5-6 on curve 92. However, it will be seen that the eifective portions of wave 32 both result in a decrease of the plate current, due to the :i-act that the voltage swing moves from the saturation zone A down along the respective limbs to cut-oil and upward again from cut-oil to the portion 29 and hence this eflective change in plate current-will have twice the frequency of the fundamental or input voltage wave. Curve 93 represents a wave having an amplitude in excess of the values represented by the cut-oil points of curve 21. Again, those portions of the wave lying in the zone A have no eil'ect upon the plate current and those portions thereof lying beyond the cut-01f values likewise are ineffective to change the plate current, while those portions lying between the termini of limbs 29 and 30 of curve 21 are practically rectangular and therefore represent higher harmonics, in some instances many orders higher than the fundamental frequency of the input voltage, the form of the output wave due to voltage 33 being shown in the output as 1-9-9.

In Fig. 3 the curve 34 showsthe variation of double frequency output 4-5-9 in Fig. 2 in rel-ation to the input. It will be noted that the double frequency outputrises rapidly for certain changes in the input then declines gently for a space and then curves downwardly on a line that is substantially asymptotic.

The-curves 3t, 92 and 33 in Fig. 2 represent,

respectively, low amplitude noise,- the desired signal amplitude, and high amplitude interference that may be either natural or might be deliberately produced spark interference sent out by an enemy attempting to blanket radio commuaratehigh amplitude interference from signal amplitudes. Here-the bias of the signal grid i3 is adjusted to a value represented by substantially the junction of portion 29 of curve 21 with the limb 28 thereof.--, In the light of the discussion of Fig. 2, it will be readily perceived that the portion l-2 of the input wave lies between the termini of limb29, and is therefore eii'ective in changing the output, whereas the portion 2-! lies between the limits of the portion 29 and does not affect the output hence the signal will appear in the output at its fundamental frequency. However, the portion 3-4 of the wave, wherein the amplitude is excessive, will affect the output only as to 'so much of the wave as lies between the termini of limb 28 and will give a substantially rectangular output 3-4 representing higher harmonics; The portion 4-! of the wave will likewise appearinthe output as higher harmonics due to so'much of the wave as-iies between the termini of limb 30, but the form thereof is seen to be much narrowed as compared to 9-4.

Fig. 5 represents an application of the present invention wherein, a continuous wave is modulated to carry two envelopes lill and ii of different amplitude by shifting from one amplitude to the other at supersonic frequency. The carrier having a frequency represented graphically by the transverselines in Fig. 5 may have a frequency of 50toj 100 times the supersonic frequency of shifting trom one amplitude to the other, which supersonic frequency may be from to kilocyoles per second. The portions of greater amplitude represented by lined elements 34 define an envelope I50 of greater amplitude lying within the cut-off limits of curve 21 while the envelope ii of lesser magnitude defined by portion 35 lies within the limits of the portion 29 of curve 21. It is apparent, from previous discussion, that if the signal grid of the tube be biased to avalue represented by dashed line 39 the envelope 50 will appear in the output at double the fundamental frequency represented by this envelope and may be made the signal bearing envelope. However, if the control grid of the tube be biased at points represented by either dashed line 31 orv dashed line 38 the envelope 59 will not appear in the output except possibly as higher harmonics while the envelope Bl will be in the output at its fundamental frequency. It is apparent that an ordinary receiver cannot discriminate betweenthese two envelopes and hence a signal carried by either one could not be interpreted but the separation of the signal bearing envelope may be readily achieved by the present invention.

Fig. 6 illustrates a different method of utilizing this invention wherein a continuous wave is mod ul-ated by, say, a 1000 cycle tone, to carry two en- 2,230,248 veiopes 39 and 40 of equal strength or db. wherein the minimum of the one substantially coin- 'cides with the maximum of the other. It is believed to be obvious how either of these envelopes may be separated from the other at the receiver when the methods disclosed herein are followed;

Fig. 7 is an application of the method of transmission set forth in connection with Fig. to the superposition of one envelope on the other as illustrated in Fig. 6, that is, the two envelopes are defined by shifting from one to the other at supersonic frequencies; a

In Fig. 8 I show how a signal envelope 4| may be blanketed by an envelope 42 of less or of substantially the same amplitude as in Fig. 6 and further blanketed by an envelope 43 of much reater strength, and yet the signal envelope 4| may be readily segregated without cross talk by the method disclosed in connection with Fig. 2.

It will be apparent to those skilled in this art that the extent of the saturation portion 29 of curve 21 may be changed eitherto increase or decrease the range represented thereby. That is, adjusting the potential divider 26 away from the ground connection raises the potential on cathode l4 and hence it requires a greater grid potential to cause the tube to pass current and likewise adjusting the potential divider I! toward ground has the same effect. Increasing the values of-resistors l8 and t9 augments the negative potential developed on these grids by interception of electrons and hence narrows the range over which the output current remains constant. I

Fig. 9 depicts the operation of my invention when adjustments of the circuit values are made to decrease the grid voltage range over which the output remains constant. It will be apparent that when the fixed grid bias has avalue represented by the low voltage limit of portion 43 of curve 44 a wave having its extreme values represented by the range designated envelope F, represented by the curve 45 will appear in the output at the fundamental frequency as indicated by |-2 in Fig. 9 and that a voltage wave having a range represented by envelope G will appear in the output at twice the fundamental frequency but the portion 45 will have greater amplitude than one portion 5-4, owing to the fact that the portion 4-5 represents a voltage change between the values defined by the termini of limb 41 of curve 44 whereas the portion 58 is due to a voltage change thatis less than the range between the terminal of limb 48 of curve 44. A wave 49 having a voltage range des-' ignated by envelope H will appear in part in harmonics as in I8 and in part as a large amplitude component 8-9.

Fig. 11 shows graphically the response to the several input voltages as the signal input increases. The space designated envelope F is that portion that appears in the output at the fundamental frequency; that designated envelope G is at twice the fundamental frequency and that designated envelope H is in the region of the higher harmonics. The curves F, G and H illustrate the changein response to each of the particular waves as the input changes and correspond respectively to the waves represented by curves 45, 46 and 49 in Fig. 9.

Fig. shows how'the form of the EgIp curve may be varied by altering .the values of resistors iBand l9 and changing the adjustment of voltage dividers 25 and 28. It is to be understood that only the general contours of these curves are shown and not the relative values; for instance, curve E may be inmicroamperes, being produced by applying cathode potential to shield grid 23 and anode Illwhile curve A may be in milliamperes when a strong positive potential is applied to the shield grid 23 and anode 20.

It should be noted that interceptor grids l6 and II should be so proportioned that they have a high absorption factor for electrons approaching them and that the load in series with each should be of relatively high impedance as compared with the internal impedance between such grid and the cathode l4.

When the network of Fig. 1 is operated to produce an Er-Ip curve like that shown in Fig. 21 with the input grid biased to operate about .the point on the descending limb thereof defined by the dotted line in-that figure, the output will be 180 degrees out of phase with the ouput that would be produced if the input grid were biased at the corresponding point on the ascending limb of the curve. This is clearly shown in Fig. 17 where the input wave 60 results in the output wave 6| when the input grid is biased about substantially the midpoint 62 of the ascending limb of characteristic curve 63. The same input wave 60, when the input'grid is radio frequency circuits and if the frequency,

the mu of the tube and internal capacity of the tube are of proper values there will be no distortion nor will instability result from the use ofthe tube with inductive networks in the input and output circuits. In this way the capacities within the tube will cause degeneration rather than regeneration as is the usual case.

If the tube be operated on an E -Ip curve like that of Fig. 19 and the input grid be biased at the point thereon defined by the dotted line, input signals of low amplitudes will work on the rising slope and be taken off at thefundamental frequency but, signals of vsuiiicient strength to pass the apex and go over to the descending slope will cause double frequency components in the output that will abstract from the incoming high peaks most of their power and the fundamental frequency taken, off will be reduced average power. Also, if the backgroundnoise is low so as'to operate only on the rising limb, the signal may be taken off the output at double frequency, provided it is held within the values represented by the lower extremities of the two limbs of the curve, while high amplitude surges pass on the response curve at both ends along the zero line and do not appear in the output networks that are tuned at fundamental and double frequency. Likewise, the third and higher harmonics will be tuned out. The harmonics, where stage during periods of excess surges, or they may be used to quench an oscillator used for heterodyning, or to cause, an automatic volume control to build up the sensitivity of succeeding radio frequency and audio frequency stages to compensate for momentary loss of signal strength during periods of static impulses when certain systems of noise reduction are used and the signal is not entirely'lost aurally. so that to a listener there will be no apparent lossin signal strength.

If operation be onacharacteristic curve such as that in Fig. 28, with the input grid biased about substantially the midpoint thereof, as indicated by the dotted line, the input signal will be converted to double frequency in the output circuits and any highinput sufllcient to swing to the horizontal zero line on either side will be lost in the subsequent tuned filter circuits. This has advantages -overthe conventional push-push doubling circuits in that a single input circuit is required and the noisesilencing properties. es-

pecially in the case of the shot gun variety of static, are superior. In radio frequency circuits 9. crystal or LC type of filter will lose or eliminate any harmonics to which the filter will not respond.

Fig. 12 disclosesa vacuum tube network adapted to utilize the outputthat is 188 degrees out of phase, as discussed in connection with Fig. 17, to counteract the degenerating action of the internal tube capacities. It will be noted that this very simple arrangement -comprises an inductance 86 and a resistance 81 in series between the cathode I4 and the ground, with a variable reslstance 88 and a capacitance 68 in parallel with the inductance and thefixed resistance, respectively, the tuned input circuit 18 being grounded.

Fig.- 13 illustrates a hookup in general similar to a regenerative network in that the cathode I4 is connected, through resistance H and capacitance 12 in parallel to a tap on the inductance 13 in tuned input 14 but, owing to the 180 degrees out of phase relation, there will be neutralizing rather than regenerating action.

Fig. 14 shows another network for utilizing the 180 degrees out of phase output for neutralizing purposes. Here the output circuit 15 is connected to cathode I4 through a resistance 18 and a point on inductance 11 of output circuit 18 is connected to a signal grid I3 through a variable capacitance 18.

Fig. 15 illustrates means for utilizing the fundamental frequency output from circuit 2|, Fig. 1, to reduce the average power of the output from a subsequent stage. As indicated by the dotted lines and the legend in Fig. 15 the tuned input circuit 18 and transformer primary 88 are fed from output circuit 2| in Fig. 1. The output circuit 8I of tube.82 is tuned to an harmonic frequency, say the third, and is coupled to the tuned input circuit 83 of tube 84. However, in series between the'input circuit 83 and grid 85 of tube 84 there is connected a circuit 88 tuned to the fundamental frequency. It is thus apparent that the input to tube 84 will be the fundamental superimposed on the chosen harmonic. These relations are shown in Fig. 16 where the curve 81 represents the fundamental and the dotted curve 88 represents the harmonic. It will be observed that the envelope is of the same form as the fundamental but the resultant is as shown by the smaller waves 88 which, while preserving the form of the fundamental envelope carry a very much reduced average power as shown by the dotted line 88. Although the amplitude of the individual waves at harmonic frequency may be present they will cause but little. if any. excitation in high L circuits into which the plate current may feed. The actual applied bias on the input circuit 85 of tube 84 will be the constant bias supplied by resistance 8i and capacitance '82 plus the momentarily added bias when the current rectifies the signal at harmonic fre- 5 quency. Resistance 83 and capacitance 84 act to store the energy and may have a time constant equal to the period of the fundamental fre-V quency, to allow the charge of rectifiedvoltage across capacitance 84 to leak ofi readily after 18 plate current cutoff is reached.

Fig. 18 discloses another type of circuit that has a neutralizing effect when operating with the output 180 degrees out of phase as above described. Here the anode 85 of tube 88 is connected back through capacitance 81 to a point on conductance 88 of tuned input circuit 88. Persons skilled in this art will readily perceive other means of usefully applying the phase relation produced according to the principles of the present invention.

Fig. 22 discloses another circuit adapted to apply the principle of the present invention. Here the transformer primary 88 is fed from 21' in Fig. 1, as in Fig. 12, and feeds the fundamental carrier frequency into tube I88 by way of grid IN and cathode I84, grid I8I being biased for Class A, B, or C operation, and thus this channel becomes the signal input for the cathode to plate circuit. The circuit comprising grid I83, cathode I84 and resistorl85 and capacitor I88 in parallel rectifies the signal put into it through transformer I81, biasing grid I83 proportionately to the maximum amplitudes of the signals received, due to the voltage drop across resistor I 85'and the energy thus stored upin capacitor I88.. The time constant of resistor I85 and capacitor I88 is preferably longer than the period of one wave. The transfer of energy through tube I88 will be controlled by the bias on grid I83, and during periods of excessive amplitude the output of the tube may be reduced substantially to zero, particularly if good shielding is effected by the R. F. grounded screen grid I88, crossfeed between the circuits of the receiver being prevented. The audio frequency component injected by the action of the rectifying circuit will be kept out of the output by radio frequency output transformer I88.

When being used in a broadcast receiver for reception of voice, etc., the time constant of I85, I88 may be longer than when used for code or continuous wave reception, where the time of a "dot may be short and there is a possibility of losing a telegraphic character unless the time constant is sufficiently small. For such purposes, the time constant may be reduced to the period of two to ten waves of the fundamental frequency presented at primary; 88, and the network of Fig. 22 then becomes suitable for code or continuous wave uses.

It is obvious that the energy inputs at 88 and I81 may be reversed without departing from the principle of operation. So long as the two input grids control the flow of electrons from the cathode to the plate, and one of these grids is constantly rectifying the signal applied thereto, at fundamental frequency or otherwise, while the other grid acts as carrier control only for the electron stream, the results will be the same.

If the time constant of I85, I86 is made less than the period of the fundamental frequency fed in at 88 and approximately the period of the harmonic fed in at I81, the results depicted in Fig. 16 will be effectively secured. In practice, 15

the capacity may be provided by the wiring and the resistance may-be of any standard type in series in the input circuit fed through I01. It is desirable that. resistor it! have such value that little current may flow during the positive half of the cycle, yet the negative charge during the negative half of the cycle in I01 may cause approximate cutoff when such negative pulses are excessive. I

When used in the audio frequency section of a receiver, the input at is audio frequency and the output of the stage is also audio, while the input through I01 may be radio or intermediate frequency. Then the individual audiofrequency waves will be chopped by the radio frequencyinput and undesirable high amplitude surges will be reduced before they enter the reproducing device, such as aloud speaker. The input through I01 may be derived from an amplitude selective device, so that the rectifying circuit supplied thereby will function only when excess amplitudes are received. From the foregoing, it is apparent that the harmonic outputfrom circuit 22, Fig. 1, may be used to control'the volume of any stage.

The invention herein described and claimed may be used and/or manufactured by or for the Government of the United States of America for governmental purposes without the payment of any royalties thereon or therefor.

I claim:

1. A method of operating a vacuum tube in a network wherein the circuit values are such that the Eg-Ip curve of the tube has a limb sloping upwardly from cutofi', a substantially straight flat-top portion, and an oppositely sloping limb descending to cut-ofl, comprising the steps of biasing said tube to operate about a value represented by substantially the midpoint of said portion whereby input voltage swings of amplitude not greater than one-half the length of said portion will'not affect the output of the tube, taking oil at double the original frequency the output of the tube resulting from input voltage values lying between the termini of said limbs, and eliminating from the said output the higher harmonic energy resulting from input voltages lying between the termini of said limbs when the extreme values of such voltages lie beyond the said cut-ofl' points.

2. A method of operating a vacuum tube in a network wherein thecircuit values are such that p the EgIp curve of the tube has a limb sloping upwardly from cutofi', av substantially straight flat-top portion, and an oppositely sloping limb descending to cut-off, comprising the steps of biasing said tube'to operate about a value represented by substantially the low voltage limit of said portion whereby input voltageswings of amplitude not greater than the'length of said portion are represented in the output of the tube at the frequency of the incoming signal, and eliminating from the said output the higher harmonic energy due to voltage swings whereof the extreme values lie beyond said cut-off points.

3. A method of operating a vacuum tube'in a network wherein the circuit values are such that the Er-Ip curve of the tube has a limb sloping upwardly from cut-oil, aasubstantially straight flat-top portion, and an oppositely sloping limb descending to cut-oil, comprising the steps of biasing said tube to operate about a value represented by substantially a limit of said portion whereby input voltage swings of amplitude not greater than the length of said portion are repquency of the incoming signal, and eliminating from the said output the higher harmonic energy due to voltage swings whereof the extreme values lie beyond said cut-oil points.

a 4. The method of separating medium amplitude radio signals from low amplitude noise and excessive amplitude disturbances accompanying said signals, in a network including a tube operating on an lit -I, curve having a limb sloping upwardly from cut-off, a substantially straight flat-top portion and an oppositely sloping limb descending tocut-oif, which comprises operating about a grid bias having a value represented by substantially the midpoint of said portion, adlusting, the values in said network so that the extremes of the low amplitude noise swings do not extend beyond the limits of said portion and the extreme values of the si nal voltage swings lie within said cut-ofl points, taking said signal from the output at twice the received frequency, and eliminating from said output the higher harmonics produced in said output by those portions of excessive amplitude swings lying between the about a grid bias having a value represented by substantially the low voltage limit of said portion, adjusting the values in said network so that the extreme values of the swings of the signal. voltage do not extend beyond the other limit of said portion and the extremes of voltage swing due to said disturbances lie beyond said cut-off points, separating said signal from said output and eliminating the higher harmonics in said output due to those portions of said excessive amplitudes lying between the termini of said limbs.

6. A method of separating radio signals from excessive amplitude disturbances accompanying said signals, in a network including a tube operating on an Eli-1 curve having a limb sloping upwardly from cut-off, a substantially straight flat-top portion and an oppositely sloping limb descending to cut-off, which comprises operating about a grid bias having a value represented by a limit of said portion, adjusting the values in said network so that the extreme values of the swings of the signal voltage do not extend beyond the other limit of said portion and the extremes of voltage swing due to said disturbances lie beyond said cut-off points, separating said signal from said output and eliminating the higher harmonics in said output due to those portions of said excessive amplitudes lying between the termini of said limbs.

7. A method of amplitude selection of radio signals, comprising the steps of biasing a vacuum said electrons to reduce the output current over a predetermined portion of the input voltage swing, whereby those portions of excess amphtude swings that appear in the output are thrown into higher harmonics, separating out the portion of the output having the chosen signal amplitude, and-eliminating from the output the said higher harmonics.

8. A method of doubling the frequency of a radio signal wave, comprising the steps of adjusting the values in a vacuum tube network so that the tube operates on an Er-Ip curve having a limb' rising from cut-oil, a substantially straight fiat-top portion and a limb descending from said portion to cut-oif, biasing the control grid of said tube to a value represented by substantially the midpoint of said portion, and taking of! as the signal those portions of the output due to input Voltages lying between the termini of said limbs. 9. A method of radio reception, comprising the steps of receiving a wave train modulated at two amplitudes which shift at supersonic frequency from one of said amplitudes to the other whereby said train carries modulation envelopes of two amplitudes, changing the envelope of greater amplitude to higher harmonics and filtering out said harmonics, and utilizing said envelope of lesser amplitude at its fundamental frequency.

10. A vacuum tube network, comprising a vacuum tube having a plurality of grids, a cathode and an anode, the elements of said tube being biased to cause said tube to operate about substantially the mid-point of the descending limb of an Eg-Ip characteristic curve having an ascending limb, a saturation current portion and a descending limb, an inductance and a resistance connected in series between said cathode and ground, a variable resistance and a capacitance in parallel with said inductance and said resistance, respectively, and a grounded tuned input operatively connected to one of said grids, whereby is counteracted the degenerating effect of the internal tube capacities.

11. A vacuum tube network, comprising a vac- 40 uum tube having a plurality of grids, a cathode and an anode, the elements of said tube being biased to cause said tube to operate about substantially the mid-point of the descending limb of an EgIp characteristic curve having an ascend- 4 ing limb, a saturation current portion and a descending limb, a grounded tuned input circuit including an inductance and a capacitance connected to said grid, a resistance and a capacitance connected in parallel to said cathode, and means connecting the other common terminal of said parallel connected resistance and capacitance to a point on said inductance.

12. A vacuum tube network, comprising a vacuum tube having a plurality of grids, a cathode and an anode, the elements of said tube being biased to cause said tube to operate about substantially the mid-point of the, descending limb of an Er-Ip characteristic curve having an ascending limb, a saturation current portion and a descending limb, a tuned input circuit connected to said grid, a resistance and a capacitance connected in parallel between said cathode and ground, means connecting said tuned input circuit to ground, a tuned output circuit including an inductance and a capacitance in parallel, a

resistance connected between said output circuit and said cathode. and a variable capacitance connected between said grid and a point on said inductance.

13. A radio network, comprising a first vacuum tube having an input circuit tuned to a fundamental frequency and an output circuit tuned to a harmonic of said fundamental frequency, a second vacuum tube having a cathode, a signal grid and an anode, means coupling the output of said first tube to the grid of said second tube including a circuit tuned to said harmonic and a circuit tuned to said fundamental connected in series, means to supply energy of said fundamental frequency to both said circuits tuned to said fundamental frequency connected to the anode of said second tube, a resistance and a capacitance in parallel between the fundamentaltuned circuit in the input of said second tube and ground, a resistance and a capacitance in parallel between said cathode and ground, a resistance connected to thesaid tuned output of said second tube and to a point between said cathode and the said resistance between said cathode and ground, and a resistance and capacitance in series between said tuned output of said second tube and the ground end of the resistance connected between said cathode and ground.

14. A vacuum tube network, comprising a vacuum tube having a plurality of grids, a cathode and an anode, the elements of said tube being biased to cause said tube to operate about substantially the mid-point of the descending limb of an EgIp characteristic curve having an ascending limb, a saturation current portion and a descending limb, a tuned input circuit including an. inductance and a capacitance connected to one of-said grids, means connectingsaid input circuit to ground, a resistance connecting said cathode to ground, a capacitance connecting said cathode to said input circuit ground connection, an output circuit connected to said anode, a resistance connecting said output circuit to said cathode, a second resistance connecting said outputcircuit to a second grid, means connecting a third grid to ground, and a capacitance connecting said anode to said one grid.

15. A method of reducing the amplitude of output energy from a vacuum tube receiving network, comprising the steps of receiving signal energy having a fundamental frequency and that may be affected by non-signal energy of excessive amplitude, deriving from the energy thus received energy at said fundamental frequency and energy at a frequency that is a higher harmonic of said fundamental frequency, utilizing said energy at PHILIP M. HAFFCKE. 

